feat(yaml): introduce YamlSyntaxError with structured position info

[tomas-zijdemans]

Introducing YamlSyntaxError mirroring XmlSyntaxError from @std/xml:

• YamlSyntaxError extends SyntaxError with line, column, offset, snippet
• parse() and parseAll() now throw it (including the multi-document case, with position pointing at the second ---)
• onWarning callback type tightened to YamlSyntaxError
• error.message format unchanged. Fully backward-compatible

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Note

Recreates #7156, which was closed automatically when I accidentally deleted the head fork. The branch is identical to the original PR head.